Precious metal composition

ABSTRACT

A homogeneous composition for forming on firing a film of precious metal, which is one or more of platinum, palladium, gold and silver, on a substrate, for instance to decorate table ware, comprises polymeric resin and a solution, in water and a co-solvent, of thiolate of the precious metal, the composition containing 3-22% by weight of the precious metal as the thiolate, and the co-solvent, resin and thiolate being such that as the composition on a substrate dries and is progressively heated in firing, the water evaporates off to leave a homogeneous composition of the resin and thiolate in the co-solvent, then the co-solvent evaporates off to leave a homogeneous composition of the thiolate in the resin, and then the thiolate decomposes to the precious metal while the resin volatilize.

This is a DIVISION of application Ser. No. 08/155,827, filed Nov. 23,1993, now U.S. Pat. No. 5,401,535 which in turn is a DIVISION ofapplication Ser. No. 07/875,412, filed Apr. 29, 1992, now U.S. Pat. No.5,281,635.

This invention relates to a composition for forming a film of preciousmetal on firing the composition, and to a method of forming a film ofprecious metal on a substrate using the composition.

Compositions which form a film of precious metal (by which is meant inthis specification one or more of platinum, palladium, gold and silver)on firing are useful particularly for decorating ceramic or glasssubstrates.

Commercially available liquids for this purpose are based on organicsolvents and are complex mixtures containing (a) a precious metalcompound which decomposes on firing to form the precious metal, (b)metal compounds, such as those of rhodium, bismuth or chromium, toimprove the colour, adhesion or brightness of the fired film and (c)resin and organic solvent to dissolve or carry (a) and (b).

It would be desirable to reduce or eliminate such organic solvent so asto reduce or eliminate any environmental hazards it presents, forinstance in the preparation, transportation or use of compositionscontaining it, for example from the fumes released on applying thecompositions or firing them. Such a task is not straightforward,however, because the compositions still of course need to be stable,apply readily and produce good films on firing. A composition which isbased on water rather than on organic solvent would have the addedadvantage that materials, such as brushes or screen printing screens orstencils, with which the composition came into contact could be cleanedwith water-based cleaning agents. In spite of these advantages, there isno water-based bright gold composition commercially available as far aswe know. A water-based composition has now been invented.

UK patent specification 2097811 A discloses a paint medium for applyingoverglaze decoration to pottery, characterised in that it comprises:

15-40% by weight of polyvinyl pyrrolidone or a mixture of polyvinylpyrrolidone and aqueous polyethylene oxide,

45-85% by weight of ethylene glycol and/or propylene glycol, andoptionally water. The specification mentions colouring matter which maybe oxides and gold and organic gold compounds.

UK specification 2216536A discloses a burnish gold compositioncontaining 10-40% by weight of gold in the form of gold powder and/or asparingly soluble gold compound, 2-20% by weight of polyvinylpyrrolidone, 3-30% by weight of an aqueous acrylate resin dispersionwith 30-60% by weight of solids content, and 5-50% by weight of water.

The present invention provides a homogeneous composition for forming onfiring a film of precious metal, which is one or more of platinum,palladium, gold and silver, on a substrate, which composition comprisespolymeric resin and a solution, in water and a co-solvent, of thiolateof the precious metal, the composition containing 3-22% by weight of theprecious metal as the thiolate, and the co-solvent, resin and thiolatebeing such that as the composition on a substrate dries and isprogressively heated in firing, the water evaporates off to leave ahomogeneous composition of the resin and thiolate in the co-solvent,then the co-solvent evaporates off to leave a homogeneous composition ofthe thiolate in the resin, and then the thiolate decomposes to theprecious metal while the resin volatilises.

The invention provides also a method of forming a film of precious metalon a substrate, which method comprises applying the composition to asubstrate and drying and firing the applied composition.

The invention also provides a transfer bearing the composition which hasbeen dried thereon.

Some of the present thiolates are novel compounds, and the inventionprovides novel compounds as such, in particular a thiolate which is agold compound of formula AuSR'' or a salt thereof, in which R'' is suchthat HSR'' represents:

4,6-dihydroxy-2-mercaptopyrimidine; or

N-(mercaptoalkanoyl)glycine whose alkan group contains 1 or 2 carbonatoms.

The invention provides also a process for preparing a novel thiolate,which process comprises reacting in a solvent a compound of formulaAuCl(SR'₂) with a thiol of formula HSR'' to form the gold compound, andin solution, the sulphide of formula SR'₂.

The invention provides also a process for preparing a novel thiolate,which process comprises reacting a tetrahaloaurate with a thiol offormula HSR'' to form the gold compound.

The invention also provides a process for preparing a novel thiolate,which process comprises reacting a thiol of formula HSR'' with a gold(I)amine complex of formula ##STR1## wherein Hal represents a halogen atomand each R''' is the same or different and represents an aliphatic,aromatic or heterocyclic group or the two R''' symbols together with theN atom represent a cyclic amine to form the gold compound.

Though the present composition still contains co-solvent, thecomposition is water-based. It can be diluted with water (ie, it iswater-thinable) and need not contain as much organic solvent asconventional liquid precious metal formulations. It can be appliedreadily, and produces good films on firing.

It is surprising that a water-soluble (not colloidal) precious metalcompound can be employed. The invention is based on the discovery thatthis can be done if the compound is a dissolved thiolate and, as thecomposition is progressively heated on firing, the precious metalcompound remains in homogeneous composition, preferably in solution,until the precious metal compound decomposes to the precious metal whileits carrier matrix volatilises.

For clarity, the invention is described in terms of heating to leave ahomogeneous composition, preferably a solution, of the precious metalthiolate in the co-solvent and then a homogeneous composition,preferably a solution, of the thiolate in the resin. It will berecognised, however, that the thiolate may change in structure duringthe healing; what is important, however, is that whatever the form ofthe subsequent precious metal compound, it remains in homogeneouscomposition, preferably in solution. Similarly, the resin may decomposeduring its volatilisation.

As the present composition is heated, a homogeneous composition ismaintained, so that the thiolate does not precipitate until itdecomposes to the precious metal while the resin volatilises. Thethiolate is in solution in the initial composition, and it is preferredthat it remain in solution during the heating until it decomposes to theprecious metal while the resin volatilises. Accordingly, it is preferredthat the composition comprises a solution, in water and a co-solvent, ofpolymeric resin and thiolate of the precious metal, the compositioncontaining 3-22 % by weight of the precious metal as the thiolate, andthe co-solvent, resin and thiolate being such that as the composition ona substrate dries and is progressively heated in firing, the waterevaporates off to leave a solution of the resin and thiolate in theco-solvent, then the co-solvent evaporates off to leave a solution ofthe thiolate in the resin, and then the thiolate decomposes to theprecious metal while the resin volatilises.

The present composition usually contains 5-85% of water. Percentagesherein are by weight. The composition can be diluted with water forapplication. The composition contains preferably 15-70%, especially40-55%, of water. It is especially advantageous that the compositioncontain a greater weight of water than the total of all organic solvent.

The precious metal usually comprises gold or platinum, especially gold.Specific instances are:

(a) gold;

(b) platinum;

(c) gold and platinum;

(d) gold and palladium;

(e) gold and platinum and palladium; and

(f) any one of (a)-(e) and silver.

Preferably, the precious metal is (a), (b), (d) or any one of these andsilver.

A thiol is an organic compound which is an analogue of an alcohol, theoxygen atom of the hydroxyl group of the alcohol having been replacedformally by a sulphur atom. A metal thiolate is a derivative of a thiolin which the hydrogen atom of its SH group is replaced formally by ametal atom. The present precious metal thiolate must form a solution inthe water/co-solvent mixture, and also be such as to fulfil the otherfunctions demanded. Usually the thiolate employed has a solubility inwater at 20° C. of at least 100, preferably 200-300, grams per liter.Preferably, the thiolate, usually in the form of a salt, is soluble inthe water alone. Advantageously, the precious metal, especially gold,thiolate starts to decompose to precious metal above 200° C.; suchthiolates tend to give bright rather than dull films after firing.Suitable thiolates are known compounds.

The thiolate can be an aliphatic, aromatic or heterocyclic thiolate,preferably containing at least one group selected from nitro groups andgroups of formula --COOH, --SO₂ OH, --OH, --CO.NH₂, --NH₂ or--O--P(O)(OH)₂, any of whose H atoms optionally being substituted, or asalt thereof. Such thiolates are discussed below with reference to theprecious metal being gold. Thus, a preferred group of thiolates are goldcompounds of formula:

    Au--S--R--H or Au--S--R--X

or a salt of either, in which:

X represents a nitro group or a group of formula --COOH, --SO₂ OH, --OH,--CO.NH₂, --NH₂ or --O--P(O)(OH)₂, any of whose hydrogen atomsoptionally being substituted; and

R represents a divalent organic group.

Although the gold compounds are described, in the conventional way, asbeing of a mononuclear formula such as Au--S--R--X, it will beunderstood that in fact they may be of polynuclear formula to satisfythe 2-co-ordinate nature of gold(I) complexes.

Any substituent in X is usually an aliphatic, aromatic or heterocyclicgroup or an optionally substituted amino group, for instance an aminogroup mono- or di-substituted by alkyl of 1-6 carbon atoms. Thealiphatic group is usually alkyl of 1-6 carbon atoms, for instanceethyl, isopropyl or n-butyl. The aromatic group is usually optionallysubstituted phenyl, any substituent usually being X as here defined,alkyl of 1-6 carbon atoms or mercapto whose hydrogen atom is optionallysubstituted by a gold atom. The heterocyclic group is usually optionallysubstituted purine, pyridine or pyrimidine, any substituent usuallybeing X as here defined, alkyl of 1-6 carbon atoms, or an aromatic groupas described above. The --CO.NH₂ or --NH₂ group may be mono- ordi-substituted, for instance by alkyl of 1-6 carbon atoms optionallysubstituted by X as here defined.

When R represents a divalent aliphatic group (ie the group attached to Sis aliphatic rather than aromatic or heterocyclic), it is usually adivalent hydrocarbon of 1-7 carbon atoms (eg alkane of 1-6 carbon atomsor toluene) optionally substituted by X as defined above, usually by oneX, or by an oxo group on a non-terminal carbon atom. When R represents adivalent aromatic group (ie the group attached to S is aromatic ratherthan aliphatic or heterocyclic), it is usually divalent benzeneoptionally substituted by X as defined above, usually by one X. When Rrepresents a divalent heterocyclic group (ie the group attached to S isheterocyclic rather than aliphatic or aromatic), it is usually divalentpyrimidine, purine or pyridine optionally substituted by X as definedabove (usually by one X) and/or by phenyl (usually by no more than onephenyl) and/or by alkyl of 1-6 carbon atoms (usually by no more than onesuch alkyl group).

X preferably represents --COOH.

In a particular embodiment, --R--X represents --CH₂ CH₂ X,--CH(CH₃)CO.NH.CH₂ X, --CH₂ CH₂ CO.NH.CH₂ X, --CH₂ C₆ H₄ --p--X, --CH₂COCH₂ CH₂ X, --CH(X)CH₂ X, --CH₂ CH(NH₂)X, --CH₂ CO.NH.CHX.CH₂.C₃ H₃ N₂,--CH₂ CO.NH.CH₂ CH₂ CO.NH.CHX.CH₂.C₃ H₃ N₂, --CH₂ CONH.CH₂ CH₂ CO.NH.CH₂CO.NH.CH₂ X, --CH₂ CH(CO.NH.CH₂ X)NH.CO.CH₂ CH₂ CH(NH₂)X or--CS.N(CH₃)CH₂ CH₂ X, in which X represents --COOH, C₆ H₄ representsdivalent benzene and C₃ H₃ N₂ represents imidazol-4-yl.

In another particular embodiment, --R--X represents --CH₂ CH₂ X or --CH₂--C₆ H₄ --p--X, in which X represents a nitro group or a group offormula --SO₂ OH, --OH or --NH₂ and C₆ H₄ represents divalent benzene.

In a further particular embodiment, --R--X represents --C₆ H₄ --p--X, inwhich X represents a nitro group or a group of formula --SO₂ OH, OH orNH₂ and C₆ H₄ represents divalent benzene.

In a yet further particular embodiment of the gold compounds of formulaAu--S--R--H or Au--S--R--X, H--S--R--H or H--S--R--X represents:

4,5-diamino-6-mercapto-2-phenylpyrimidine,

2-amino-6-mercapto-8-methylpurine,

2-mercapto-4-amino-pyridopyrimidine,

2-mercaptopyridine,

2-mercaptobenzoxazole,

2-mercaptobenzimidazole,

5-mercapto-1-methyltetrazole,

4-mercaptopyridine,

2-mercapto-5-nitrobenzimidazole, or

4-amino-6-hydroxy-2-mercaptopyrimidine.

The sulphur atom which links to the gold atom in a preferred class ofgold compounds is linked to the same carbon atom, or is separated by onecarbon atom from the carbon atom, which bears a substituted amido group,so that these compounds contain the group ##STR2##

Specific examples of this class are gold compounds of formulaAu--S--R--H or Au--S--R--X which are such that H--S--R--H or H--S--R--Xrepresents:

N-(2-mercaptopropionyl)glycine,

N-(3-mercaptopropionyl)glycine,

N-(2-mercaptoacetyl)glycine,

2-mercaptoacetyl-L-histidine or

2-mercaptopropionyl-L-histidine.

Another preferred class of gold compounds is of formula: ##STR3## inwhich n represents O or an integer of at least 1 and Y represents ahydrogen atom or an alkanoyl group of 2-6 carbon atoms which mayoptionally bear a single amino(carboxy)methyl group. Theamino(carboxy)methyl group, when present, is usually on the terminalcarbon atom of the alkanoyl group. Usually n represents O or an integerof 1-4.

Specific examples of this class are gold compounds of formulaAu--S--R--H or Au--S--R--X which are such that H--S--R--H or H--S--R--Xrepresents:

acetylcysteine,

glutathione or

L-cysteine.

A specific preferred gold thiolate is N-(2-mercaptopropionyl)glycinegold (I), of formula: ##STR4## which may alternatively be written in theZwitterion form: ##STR5## though the thiolate is believed to bepolynuclear in accordance with the discussion above.

With regard to this gold thiolate, we refer to UK patent specification2047672A. The reference is not concerned with film formation, but withpreparing a ^(195m) Au-containing liquid by adsorbing ^(195m) Hg onto acertain adsorption agent and then eluting the daughter radioisotope^(195m) Au. Table I on page 5 lists combinations of adsorption agentsand eluants, and among these eluants is an aqueous solution ofmercaptopropionyl glycine. There is no specific disclosure of the use ofthis eluent, and no specific data on the present gold thiolate. ExampleII describes a series of experiments in which certain liquids containing¹⁹⁸ Au are passed through certain adsorption agents. The Exampleexplains that the isotope ^(195m) Au was employed instead of thedaughter radioisotope ^(195m) Au because inter alia of the shorthalf-life of ^(195m) Au. The Example states that the liquids wereprepared by dissolving 1-3 μg of gold containing ¹⁹⁸ Au in 1 ml of anaqueous solution having a pH of about 5-6 and containing 0.001-0.1 molarof one of a Table of what it calls gold ion complexing agents. One ofthe agents is mercaptopropionyl glycine, and the Table records thatemploying it with the adsorption agent SiO₂ /Ag left 88% of the ¹⁹⁸ Aunot adsorbed. We believe that employing mercaptopropionyl glycine as theagent would not give the present gold thiolate, but gold having moietiesstrongly adsorbed on the surface of the metal, and hence not having onegold atom per N-(2mercapto-propionyl)glycine ligand as the present goldthiolate does. In any event, the gold materials suggested or disclosedin this reference are radioactive. They would decay to becomeprogressively less radioactive, but some radioactivity would remain. Thegold in the present gold thiolates is preferably naturally occurringgold, which is non-radioactive. Thus, it is preferred that the gold inthe present N-(2-mercaptopropionyl)glycine gold(I) or salt thereof benon-radioactive (and hence is not a mixture of the non-radioactive formwith the radioactive form). It is preferred that all the presentthiolates, not only N-(2-mercaptopropionyl)glycine gold(I) or a saltthereof, be non-radioactive.

The optionally substituted sulpho group (--SO₂ H) which X can representin the present gold thiolates introduces more sulphur, which generallyforms sulphur oxides on firing the composition to form a film, and henceit is less preferred that X has this value. Nitro and optionallysubstituted hydroxyl groups tend to make the thiolate less water-solubleand less stable to reduction than is ideal, and hence it is lesspreferred that X has either of these values. Generally, it is mostpreferred that X represents optionally substituted carboxy.

The thiolate can be in the form of a salt. Salts can be formed withbases by reason for instance of the carboxy or sulpho group which X canrepresent. A thiolate which forms a salt with a base can be employed inbasic solution which forms the salt therein. Any salt is preferably withammonia or an organic base particularly an optionally substituted amine,any substituent being for instance alkyl of 1-6 carbon atoms optionallysubstituted by X as defined above. Particular optionally substitutedamines of interest are triethylamine, ethanolamine, diethanolamine,triethanolamine, pyridine, morpholine or diisopropylamine, especiallythe first four of these.

Where the present thiolates are novel, they can be made in ways known inthemselves. Where the thiolate is a salt, it can be made for instance byreaction with a base.

When the present thiolate is a gold compound of formula AuSR'' or a saltthereof in which R'' is such that HSR'' representsN-(mercaptoalkanoyl)glycine whose alkan group contains 1 or 2 carbonatoms, it can be seen that the N-(mercaptoalkanoyl)glycine is:

N-(2-mercaptoacetyl)glycine,

N-(3-mercaptopropionyl)glycine, or

N-(2-mercaptopropionyl)glycine.

The last mentioned is particularly outstanding.

The present thiolates can be prepared by the general procedure of A KAl-Sa'ady et al, Inorganic Synthesis, volume 23, 191(1985), representedin the case of gold by the general equations:

     AuCl.sub.4 !.sup.- +2SR'.sub.2 → AuCl(SR'.sub.2)!+R'.sub.2 SO+2HCl+Cl.sup.-

     AuCl(SR'.sub.2)!+HSR''→AuSR''+HCl+SR'.sub.2

Au(III) in the form of tetrachloroaurate anion is reduced to Au(I) by awater-soluble sulphide of formula SR'₂. Each R' radical is the same ordifferent and preferably represents methyl, ethyl or 2-hydroxyethyl.Preferably the water-soluble sulphide is ethyl 2-hydroxyethyl sulphide.Addition of the appropriate thiol ligand via the thiol HSR'', usually atambient temperatures, yields a gold thiolate, generally as a whiteprecipitate though in the case of mercaptosuccinic acid as a yellowsolution. The reaction solvent is usually water, ethanol or isopropanol,preferably water so that soluble inorganic salts (for example NaCl) canbe extracted by water-washing. Yields of the gold thiolates can be ashigh as 80% or more (based on gold), with the remainder soluble inwater. The gold thiolates produced can be dissolved in water by theaddition of a base, usually an organic base, for instance an optionallysubstituted amine, to form the corresponding salt, in this instance asubstituted ammonium salt.

The present gold thiolates can also be prepared by reacting atetrahaloaurate (particularly a tetrachloroaurate or tetrabromoaurate)with the thiol of formula HSR'' to form the gold compound.

The present gold thiolates can also be prepared by reacting the thiol offormula HSR'' with a gold(I) amine complex of formula ##STR6## whereinHal represents a halogen atom (preferably a chlorine or bromine atom)and each R''' is the same or different and represents an aliphatic,aromatic or heterocyclic group, or preferably the two R''' symbolstogether with the N atom represent a cyclic amine (such as benzotriazoleor pyridine) to form the gold compound. The aliphatic, aromatic orheterocyclic groups which R''' may represent can be such groups as arediscussed above as substituents in X.

The present thiolate can be formed in situ in the present composition,for instance by forming a salt therein, for example as described above.

The present composition contains 3-22% of precious metal, as thethiolate. The composition preferably contains 8-12% of the preciousmetal, especially gold, as the thiolate. The composition usuallycontains 10-40%, preferably 14-26%, of the thiolate, especially goldthiolate.

The polymeric resin must form a homogeneous composition with the otheringredients, and also be such as to fulfil the other functions demanded.The resin preferably forms a solution in the water/co-solvent mixture,though it may form a dispersion or emulsion therein. Preferably, theresin is soluble or forms a clear dispersion, in the water alone.Suitable resins are known materials. Polyacrylic, polymethacrylic,polyvinyl pyrrolidone, polycellulose ether, polyamide, polyethyleneglycol, polyester, polyacrylamide, polyvinyl acetate, polyvinyl alcohol,alkyd, polyamine or polyurethane, resins or mixtures thereof can beemployed. The resin can be in the form of a co-polymer or graft polymer.The resin can contain solubilising groups, particularly carboxylateand/or hydroxy and/or amino groups, especially carboxylate groups, toenhance the water solubility of the resin. Preferably, the resincomprises one or more, usually one, of polyvinyl pyrrolidone resins,polymethacrylic resins and polycellulose ether resins, for instance oneor more, usually one, of polymethacrylic resins and polycellulose etherresins. Resins tend to volatilise steadily over a range of temperaturesrather than all the substance volatilising at the same temperature.Preferably, the present resin is chosen such that the majority at leastof it volatilises in the range 200°-550° C. Preferably, the resincomprises polyvinyl pyrrolidone, polymethacrylic or polycellulose etherresin the majority of which volatilises in this range.

The resin must satisfy the present requirements that on firing, thethiolate forms a homogeneous composition, preferably a solution, in it,and then the thiolate decomposes to the precious metal while the resinvolatilises. For instance, if gold mercaptosuccinic acid, whichdecomposes to gold around 150° C., is employed in a compositionanalogous to the present compositions but whose resin starts toevaporate around 200°-300° C., then on firing, a homogeneous composition(for instance a solution) of the thiolate in the resin is not formed.Instead, a premature dispersion of gold in the resin forms. A dull filmresults. However, the same resin can be employed successfully to producebright gold films with a thiolate with which it does satisfy theserequirements; and the same thiolate can be employed successfully toproduce bright gold films with a resin with which it does satisfy theserequirements. For example, N-(2-mercaptopropionyl)glycine gold(I) startsto decompose around 200° C. and decomposes to gold around 280°-300° C.;it can be employed successfully to produce bright gold films with resinswhich volatilise around 280°-300° C. and which would be unsuccessfulwith gold mercaptosuccinic acid.

The present composition can contain less than 5% of the resin butusually contains 5-45%, especially 7-13%, of the resin. The weight ratioof resin solid to precious metal, especially gold, in the thiolate canbe for instance at least 1:1, eg at least 1.2:1, for instance at least1.5:1, as these tend to produce better, "scum-free", and brighter filmson firing. Satisfactory bright films have been obtained, however, usingratios from 0.1:1 to 1.5:1. A typical composition has a ratio of 1:1.

The co-solvent must, with the water, dissolve the precious metalthiolate and form a homogeneous composition, preferably a solution, withthe resin, and be such as to fulfil the other functions demanded. Theco-solvent is preferably of boiling point between 100° and 280° C., forinstance between 100° and 220° C. A preferred group of co-solvents arethe water-miscible alcohols, ethers or esters, especially thewater-miscible alcohols. Preferably, the co-solvent comprises1,2-propanediol, 1,3-propanediol, n-butanol, tetrahydrofuran acetate,glycerol, glycerol diacetate or ethyl lactate, especially1,2-propanediol, 1,3-propanediol or ethyl lactate. The particularco-solvent can affect the brightness of the films on firing.

The composition usually contains 5-45%, preferably 15-30%, for instance25-30%, of the co-solvent.

It will be appreciated that particular ingredients and their amountsdepend on each other and that any candidate material can readily betested. For instance, it has been found that when the resin is acellulosic resin the content of the co-solvent in the composition ispreferably 25-30%, while when the resin is an acrylic or methacrylicresin the content of the co-solvent in the composition is preferably15-30%.

The composition can contain minor metal compound additives, such ascompounds of palladium (for example palladium sulphate), rhodium,bismuth or chromium, generally used in combination, to improve thecolour, adhesion or brightness of the fired film, particularly onpottery. Minor metal compound additives for the film particularly onglass can be for instance compounds of silver, vanadium or silicon. Forbright films, the additives are usually in solution in the composition.They may be present, for instance, in a total content of 0.01-5%,usually 0.1-0.4%, of the composition. Such additives and their amountsand function are known in themselves for use in conventional liquidprecious metal compositions based on organic solvents.

Known minor metal additive compounds which are soluble in the presentmixture of water and co-solvent can be employed, for example chromiumtrioxide or Cr₂ (SO₄)₃ !; ammonium bismuth citrate or bismuth lactate;RhCl(ox)(NH₃)!, Rh(gly)₃ !, Rh₂ (acetate)₄ !, Na₃ Rh(NO₃)₆ ! orRh(acac)CO!; Me₃ SiOMe; or VO(acac)₂ !; wherein acac representsacetylacetonate, ox oxolate and gly glycinato. In a particularembodiment, the rhodium, bismuth or silver is employed as the samethiolate as is the precious metal, for instance AgN-(2-mercapto-propionyl)glycine. Thus, where the thiolate of theprecious metal is a gold compound of formula Au--S--R--X or a saltthereof, the rhodium minor metal additive can be of formula Rh(SRX)₃ !.

The composition must be homogeneous, for ease of application etc. Toimprove the rheology of the composition for application and to controlthe drying of the applied film, the composition can contain additionalsolvent, such as a water-miscible alcohol for instance isopropanol orn-hexanol, a water-miscible ketone for instance butanone, or awater-miscible solid for instance 1,6-hexandiol. The content ofadditional solvent is preferably 5-10% of the composition. Thecomposition can contain surfactant, for instance in a content of0.05-0.5%, for example to improve the brushing properties of thecomposition.

A particular composition according to the invention consists essentiallyof:

    ______________________________________                                        Polyvinyl pyrrolidone                                                                           7-13%                                                       Gold thiolate    14-26%                                                       1,2-propanediol  15-30%                                                       Water            40-55%                                                       Isopropanol       5-10%                                                       Minor metal additives                                                                          0.1-0.4%                                                     ______________________________________                                    

A preferred composition according to the invention consists essentiallyof:

    ______________________________________                                        Polyvinyl pyrrolidone                                                                           7-13%                                                       Gold thiolate    14-26%                                                       1,2-propanediol  15-30%                                                       Ethyl lactate    15-30%                                                       Water            30-55%                                                       Isopropanol       5-10%                                                       Minor metal additives                                                                          0.1-0.4%                                                     ______________________________________                                    

The present composition may be UV-curable by reason of the polymericresin being UV-curable, or may be cured thermally by reason of thepolymeric resin being thermosetting.

The present composition can be applied to substrates and fired, each inways known in themselves and this is advantageous as conventionalequipment and techniques can be employed. The substrate on which firingoccurs is preferably ceramic, such as porcelain or china, or glass. Thecomposition can be applied directly to the substrate. Alternatively, thecomposition can be applied in the desired pattern (for instance, adecorative pattern) to a transfer substrate and dried thereon to form atransfer; the transfer can then be used in the usual way to transfer thepattern to the substrate, such as a ceramic substrate, on which it isfired. The composition can be applied for instance by brushing, by usinga steel wheel or neoprene wheel or by printing, for example ink jetprinting (otherwise known as inject or injection printing) butpreferably screen printing. Particular components in the compositionsuit particular application procedures and the particular compositionemployed depends on the particular application procedure, bearing inmind for instance the desired rheology. The applied composition isusually dried, for instance at ambient temperature, before firing. Itcan, however, be dried as part of the heating sequence for firing. Thepresent compositions are usually fired at 200°-900° C., for instance460°-650° C. on a glass substrate, 650°-900° C. on a ceramic substrate,and 200°-350° C. on a plastic substrate.

The present composition produces good films. Precious metal films fallinto three classes, burnished, self-burnished and bright. Compositionsfor forming burnished films, which--as the name implies--need to beburnished to produce the desired film, contain precious metal powder, orin some cases, insoluble precious metal compound. The fired filmsusually contain a greater weight of precious metal than do those ofbright films but are dull in appearance, exhibiting no significantreflectivity. After firing, they are burnished by mechanical abrasion,for instance with sand and water, to give a matt appearance. Morerecently, self-burnished films have been introduced. These containinsoluble matting agent which gives the fired film the appearance anddurability of a burnished film without the necessity of burnishing.Compositions for forming bright films contain soluble precious metalcompound. The fired films are very reflective, often having"mirror-like" properties. The present composition can be of any of thesetypes. Preferably, it produces a bright film on firing. The compositioncan contain in addition, however, precious metal powder or insolubleprecious metal compound to produce a burnished film on firing and thenburnishing. Alternatively, the present composition can contain inaddition an insoluble matting agent to produce a self-burnished film onfiring.

The present composition is preferably used to decorate the substrate,for instance to produce decorative table ware. The composition can beused, however, for electronic applications, for instance to lay downelectrically conductive pathways on insulating materials.

The invention is illustrated by the following Examples, in which therhodium complex unless otherwise stated is of formula Rh(acac)CO whereinacac represents the acetylacetonate group, the polycellulose ether resinis "Natrasol" resin supplied by Aqualon Limited, United Kingdom, thepolymethacrylic resin is "Versicol K11" supplied by Allied Colloids,Bradford, United Kingdom, the polyvinyl pyrrolidone is supplied byAldrich Chemicals, United Kingdom and the compositions used for filmformation unless otherwise indicated are all solutions. Unless otherwisestated, these compositions were brushed onto glazed china and thenheated to a maximum of 780° C. during a one hour firing cycle, with aten minute soak at maximum temperature.

EXAMPLE 1

N-(2-mercaptopropionyl)glycine gold(I) was prepared in a two-necked 1000ml round-bottomed flask as follows:

A solution of sodium tetrachloroaurate (35 g) in distilled water (200ml) was slowly added to a solution of ethyl 2-hydroxyethyl sulphide(20.5 g) in distilled water (200 ml). Solid addition ofmercaptopropionylglycine (5.21 g) followed by vigorous stirring yieldedgold mercaptopropionylglycine as a white solid (yield, based on gold,85%).

Analysis: Theory: C, 16.8 N, 3.9 Found: C, 16.15 N, 3.65% MeltingPoint=230° C. (with decomposition).

COMPARATIVE EXAMPLES 1-3

In an analogous way to Example 1, there were prepared:

gold L-cysteine;

gold 2-mercaptoethanesulphonic acid; and

gold 2-mercaptopyridine;

each in a yield, based on gold, in excess of 80%.

COMPARATIVE EXAMPLE 4

Gold mercaptosuccinic acid was produced in an analogous way to Example 1except that isopropanol was used instead of water and that the goldcomplex was soluble in the isopropanol and hence was isolated byevaporation of the solvent under reduced pressure (0.1 Torr) and thentrituration of the resulting oil with diethyl ether to produce a whiteprecipitate (yield, based on gold, in excess of 80%).

COMPARATIVE EXAMPLE 5

In an analogous way to Example 1, there was prepared:

gold 4-hydroxythiophenol (Analysis for C: Theory 22.3% Found 21.8%)

in a yield, based on gold, in excess of 80%.

EXAMPLE 2

In an analogous way to Example 1, there was prepared:

gold 4,6-dihydroxy-2-mercaptopyrimidine;

(Analysis for C: Theory 13.9% Found 13.2%)

in a yield, based on gold, in excess of 80%.

COMPARATIVE EXAMPLE 6

The following composition was obtained by mixing the ingredients:

    ______________________________________                                                            Parts                                                     ______________________________________                                        Polycellulose ether resin                                                                           10                                                      Bis(ethylenediamine)gold trichloride                                                                22 (10.1 parts gold)                                    1,3-Propanediol       15                                                      Water                 38                                                      Isopropanol           15                                                      Rhodium complex       0.05                                                    Chromium trioxide     0.05                                                    ______________________________________                                    

In these Examples, parts are by weight and the resin contents are basedon their dry weights.

The composition was brushed onto china and heated to a maximum of 780°C. for 10 minutes during a one hour firing cycle. A dull and pitted filmresulted. The film was not adherent; gentle rubbing removed it.

It can be seen that the gold compound is not the present gold thiolate.

COMPARATIVE EXAMPLE 7

The following composition was prepared and used in an analogous way toComparative Example 6:

    ______________________________________                                                           Parts                                                      ______________________________________                                        Polymethacrylic resin                                                                              11                                                       Gold mercaptosuccinic acid                                                                         18 (11 parts gold)                                       1,3-Propanediol      15                                                       Water                40                                                       Isopropanol          15                                                       Rhodium complex      0.05                                                     Chromium trioxide    0.05                                                     ______________________________________                                    

A dull, adherent, film resulted. The dull nature of the film was due topremature decomposition of the thiolate during firing to give aninsoluble gold product.

COMPARATIVE EXAMPLES 8-10

Compositions were prepared and used in an analogous way to ComparativeExample 7 by substituting for its gold thiolate the following goldthiolate, in an amount corresponding to 10 parts of gold:

Gold cysteine (Comparative Example 8)

Gold mercaptobenzoic acid (Comparative Example 9)

Gold 2-mercaptopyridine (Comparative Example 10)

In each case, a dull film resulted. The dull nature of the film was dueto premature decomposition of the thiolate during firing to give aninsoluble gold product. The film was not adherent; gentle rubbingremoved it.

COMPARATIVE EXAMPLE 11

The following composition was prepared and used in an analogous way toComparative Example 6:

    ______________________________________                                                           Parts                                                      ______________________________________                                        Polycellulose ether resin                                                                          17                                                       Gold thiobarbituric acid                                                                           17 (10 parts gold)                                       1,3-Propanediol      15                                                       Ethyl lactate        15                                                       Water                20                                                       Isopropanol          15                                                       Triethylamine        1                                                        Rhodium complex      0.05                                                     Chromium trioxide    0.05                                                     ______________________________________                                    

A dull and pitted film resulted. The dull nature of the film was due topremature decomposition of the thiolate during firing to give aninsoluble gold product. The film was not adherent; gentle rubbingremoved it.

EXAMPLE 3

The following composition was prepared and used in an analogous way toComparative Example 6:

    ______________________________________                                                            Parts                                                     ______________________________________                                        Polymethacrylic resin 10                                                      N-(2-mercaptopropionyl)glycine gold(I)                                                              18.5 (10 parts gold)                                    1,3-Propanediol       15                                                      Water                 40                                                      Isopropanol           15                                                      Triethylamine         1                                                       Rhodium complex       0.05                                                    Chromium trioxide     0.05                                                    ______________________________________                                    

A bright, adherent, film resulted.

It can be seen that the composition contains 10% gold as the thiolate.

The composition was also was also applied by steel wheel and by neoprenewheel. In both cases, it gave the same result as the brushing.

EXAMPLES 4-6

Example 3 was followed except that for its 1,3-propanediol there weresubstituted 15 parts of tetrahydrofuran acetate (Example 4), glyceroldiacetate (Example 5) or ethyl lactate (Example 6). In each case, theresulting gold film was adherent and essentially bright.

The composition was applied also by steel wheel, and also neoprenewheel. In each case, a bright, continuous film was obtained on firing.

EXAMPLES 7-9

The following compositions were prepared and used in an analogous way toComparative Example 6:

EXAMPLE 7

    ______________________________________                                                            Parts                                                     ______________________________________                                        Polycellulose ether resin                                                                           10                                                      N-(2-mercaptopropionyl)glycine gold(I)                                                              18.5 (10 parts gold)                                    1,2-Propanediol       15                                                      Ethyl lactate         15                                                      Water                 40                                                      Triethylamine         1                                                       Rhodium complex       0.05                                                    Chromium trioxide     0.05                                                    ______________________________________                                    

EXAMPLE 8

    ______________________________________                                                            Parts                                                     ______________________________________                                        Polyvinyl pyrrolidone resin                                                                         12                                                      (molecular weight 30,000)                                                     N-(2-mercaptopropionyl)glycine gold(I)                                                              16.5 (9 parts gold)                                     1,2-Propanediol       15                                                      Ethyl lactate         25                                                      Water                 30                                                      Triethylamine         1                                                       Rhodium complex       0.05                                                    Chromium trioxide     0.05                                                    Ammonium bismuth citrate                                                                            0.05                                                    ______________________________________                                    

EXAMPLE 9

    ______________________________________                                                            Parts                                                     ______________________________________                                        Polyethylene glycol resin                                                                           10                                                      (molecular weight 20,000)                                                     N-(2-mercaptopropionyl)glycine gold(I)                                                              18.5 (10 parts gold)                                    1,2-Propanediol       15                                                      Glycerol              5                                                       Ethyl lactate         10                                                      Water                 40                                                      Rhodium complex       0.05                                                    Chromium trioxide     0.05                                                    ______________________________________                                    

In each case, a bright, adherent film resulted.

COMPARATIVE EXAMPLE 12

The following composition was obtained and used in an analogous way tothat of Comparative Example 6:

    ______________________________________                                                           Parts                                                      ______________________________________                                        Polyvinyl pyrrolidone                                                                              91                                                       Gold-2-mercaptobenzimidazole                                                                       182                                                      Water                273                                                      1,2-Propanediol      91                                                       Ethyl lactate        227.5                                                    Ammonia solution     45.5                                                     (specific gravity 0.88)                                                        Rh.sub.2 (acetate).sub.4 !                                                                        1                                                                             811                                                      ______________________________________                                    

A dull non-adherent film resulted from the firing of the composition.This was a result of the low solubility of the gold compound duringapplication (brushing) of the composition, resulting in a heterogeneouscomposition on heating. It was estimated that only about 5% of thegold-mercaptide was dissolved in the initial medium.

COMPARATIVE EXAMPLES 13-19

Compositions were prepared and used as in Comparative Example 12 bysubstituting for its gold compound the following:

13. Gold-5-mercapto-1-methyltetrazole

14. Gold-2-mercapto-5-nitrobenzimidazole

15. Gold-4-aminothiophenol

16. Gold-4-hydroxythiolphenol

17. Gold-mercaptoacetic acid

18. Gold-2-mercaptopropionic acid

19. Gold-mercapto-2,3-hydroxypropane.

The resultant films were all unsatisfactory. The compositions ofComparative Examples 13-15 were unsatisfactory for the same reason asthat of Comparative Example 12, resulting in dull and particulate films,often with severe pitting. The compositions of Comparative Examples16-19 prematurely decomposed to gold powder during the production or useof the liquid gold compositions, resulting in heterogeneouscompositions. The gold compounds of Comparative Examples 16-19 and theirgeneral classes are inherently unstable in the presence of organic baseand therefore are not useful for producing bright gold films from waterbased media.

EXAMPLE 10

A water-based liquid gold composition that has a resin solid to goldratio of 1:10 was prepared by mixing:

    ______________________________________                                                             Parts                                                    ______________________________________                                        Polycellulose resin    9.1                                                    ("Natrasol 250GR" sold by Aqualon, UK)                                        N-(2-mercaptopropionyl)glycine gold(I)                                                               182                                                    Water                  273                                                    1,2-propanediol        91                                                     Ethyl lactate          182                                                    Triethylamine          45.5                                                    Rh.sub.2 (acetate).sub.4 !                                                                          1                                                                             783.6                                                  ______________________________________                                    

It can be seen that the composition contains 1.2% of the resin byweight.

The resulting film was bright and continuous, after gently washing offwith water the low amount of inorganic deposit left on the film and thendrying with tissue paper.

EXAMPLE 11

A composition was prepared and used as in Example 10 by substituting forits gold compound gold-acetylcysteine. It gave the same result.

EXAMPLE 12

A water-based liquid gold composition containinggold-(γ-L-glutamyl-L-cysteinyl-glycine), otherwise calledgold-glutathione, was prepared by mixing:

    ______________________________________                                                        Parts                                                         ______________________________________                                        Polyvinyl pyrrolidone                                                                            72                                                         Gold-glutathione  172                                                         Water             290                                                         1,2-propanediol    72                                                         Ethyl lactate      91                                                         Triethylamine      68                                                          Rh.sub.2 (acetate).sub.4 !                                                                      1                                                                            766                                                         ______________________________________                                    

The resulting film was bright and continuous.

EXAMPLE 13

A water-based liquid gold composition containing gold and palladium toproduce a metallic (grey) fired appearance was prepared by mixing:

    ______________________________________                                                             Parts                                                    ______________________________________                                        Polyvinyl pyrrolidone  50                                                     N-(2-mercaptopropionyl)glycine gold(I)                                                               100                                                    Palladium sulphate     15                                                     Water                  150                                                    1,2-prepanediol        50                                                     Ethyl lactate          100                                                    Triethylamine          25                                                     Polysulphide solution  12.5                                                    RhCl(ox)(NH.sub.3).sub.4 !                                                                          1                                                                             503.5                                                  ______________________________________                                    

ox=oxalato; polysulphide solution=a 15% ammonium polysulphide solutionin water (obtained from BDH, United Kingdom). The resulting film wasbright and continuous in appearance.

EXAMPLE 14

A water-based liquid gold composition suitable for fast and slow firingcycles was prepared by mixing:

    ______________________________________                                                             Parts                                                    ______________________________________                                        Polyvinyl pyrrolidone  30                                                     N-(2-mercaptopropionyl)glycine gold(I)                                                               100                                                    Water                  240                                                    1,2-propanediol        30                                                     Ethyl lactate          125                                                    n-Butanol              5                                                      Triethylamine          25                                                     Polysulphide solution  12.5                                                    RhCl(ox)(NH.sub.3).sub.4 !                                                                          1                                                      Cr.sub.2 (SO.sub.4).sub.4                                                                            1.9                                                     Bi-citrate!           2.5                                                                           572.9                                                  ______________________________________                                    

ox=oxalate; Bi-citrate was purchased from BDH, United Kingdom, asammonium bismuth citrate. The polysulphide solution was as in Example13. Reflective and continuous films were obtained from the followingfiring profiles:

Fast firing: 40 minute cycle; maximum temperature=805° C.; 2 minutessoak.

Slow firing: 200° C. per hour; maximum temperature=750° C.; 10 minutessoak.

The same composition was brushed onto porcelain ware to produce a brightand continuous film using the same firing cycle as Comparative Example6. The same composition brushed onto porcelain ware was fired to amaximum temperature of 900° C.; 40 minute cycle; 2 minutes soak, toproduce a bright and continuous film.

EXAMPLE 15

A water-based liquid gold composition suitable for application toglassware was prepared by mixing:

    ______________________________________                                                              Parts                                                   ______________________________________                                        Polymethacrylic resin   100                                                   ("Versicol K11" from Allied Colloids, UK)                                     N-(2-mercaptopropionyl)glycine gold(I)                                                                200                                                   Water                   380                                                   1,3-propandiol           20                                                   2-propanol              100                                                    Rh(CO)(acac)!.sub.2     2                                                    CrO.sub.3                1                                                    Me.sub.3 SiOMe           1                                                    Silver-N-(2-mercaptopropionyl)glycine                                                                  30                                                                           734                                                   ______________________________________                                    

The composition was brushed onto glassware, and fired for one hour intotal to a maximum temperature of 620° C., 10 minutes soak. A bright andcontinuous film resulted.

EXAMPLE 16

A water-based liquid gold composition suitable for ink-jet printing andhaving a high water concentration was prepared by mixing:

    ______________________________________                                                             Parts                                                    ______________________________________                                        Polyvinyl pyrrolidone  74                                                     N-(2-mercaptopropionyl)glycine gold(I)                                                               59                                                     Water                  314.4                                                  Butanone               46                                                     n-Butanol              39                                                     Triethylamine          8                                                      Polysulphide solution  10                                                      Rh(NH.sub.3).sub.6 !Cl.sub.3                                                                        1                                                       Bi-lactate!           1.4                                                                           614.4                                                  ______________________________________                                    

The composition of Example 16 contains 61% water. The polyvinylpyrrolidone resin has a molecular weight=10,000 daultons. Thepolysulphide solution is as in Example 13. The composition was sprayedby an ink-jet sprayer onto flat tiles and fired to produce a bright andcontinuous gold film.

EXAMPLE 17

A water-based liquid gold composition that is suitable forscreen-printing was prepared by mixing:

    ______________________________________                                                             Parts                                                    ______________________________________                                        Polycellulose resin    45                                                     N-(2-mercaptopropionyl)glycine gold(I)                                                               181                                                    Water                  454                                                    1,2-propanediol        181                                                    Glycerol               91                                                     Ethyl lactate          45                                                      Rh.sub.2 (acetate).sub.4 !                                                                          1                                                       Cr.sub.2 (SO.sub.4).sub.2 !                                                                         1.8                                                                           999.8                                                  ______________________________________                                    

The composition contains 4.5% resin as solid. The composition wasprinted through a screen of mesh size 305 μ onto glazed tile. The firedfilm was bright and continuous. A low amount of inorganic deposit lefton the film after firing was gently washed with water and then driedwith tissue paper, to give a bright and continuous film.

EXAMPLE 18

A water-based liquid gold composition which contains gold powder toproduce a burnished effect, and which is suitable for screen-printingwas prepared by mixing:

    ______________________________________                                                             Parts                                                    ______________________________________                                        Polycellulose resin     62                                                    Gold powder             400                                                   N-(2-mercaptopropionyl)glycine gold(I)                                                                250                                                   Water                  1000                                                   1,2-propanediol         250                                                   Ethyl lactate           62                                                    Glycerol                125                                                   Triethylamine           62                                                     Rh.sub.2 (acetate).sub.4 !                                                                            1                                                                           2212                                                   ______________________________________                                    

The composition was applied in an analogous way to that of Example 17.The resulting film was abraided to produce an off-bright gold effect.

EXAMPLE 19

In the same way as described for the composition of Example 19, abrushing burnish gold was prepared by simply reducing the polycelluloseresin concentration to 25 parts.

EXAMPLE 20

In an analogous way to Example 1, there was preparedgold-N-(3-mercaptopropionyl)glycine in a yield, based on gold, in excessof 80%.

Analysis: Theory: C, 16.8 N, 3.9 Found: C, 16.35 N, 3.55%

I claim:
 1. A transfer, comprising: a transfer substrate and dried thereon a composition which comprises, polymeric resin and a solution, in water and a co-solvent, of a thiolate of a precious metal selected from the group consisting of one or more of platinum, palladium, gold and silver, the composition before drying containing 3-22% by weight of the precious metal as the thiolate, and the co-solvent, resin, and thiolate being such that as the composition on a substrate dries and is progressively heated in firing, the water evaporates off to leave a homogeneous composition of the resin and thiolate in the co-solvent, then the co-solvent evaporates off to leave a homogeneous composition of the thiolate in the resin, and then the thiolate decomposes to the precious metal while the resin volatilizes.
 2. The transfer of claim 1 wherein the composition contains 5-85% by weight of water.
 3. The transfer of claim 1 wherein the composition contains 40-55% by weight of water.
 4. The transfer of claim 1 wherein the precious metal of said composition comprises gold.
 5. The transfer of claim 4 wherein the thiolate prior to drying the composition is selected from the group consisting of a gold compound of formula Au--S--R--H, a gold compound of formula Au--S--R--X and a salt of either of these gold compounds, wherein X is selected from the group consisting of a nitro group, --COOH, --SO₂ OH, --OH, CONH₂, --NH, and --O--P(O)(OH)₂, any of whose hydrogen atoms optionally being substituted with members selected from the group consisting of aliphatic, aromatic, and heterocyclic groups and optionally substituted amino groups; and R represents a divalent organic group.
 6. The transfer of claim 5 wherein the gold compound of formula Au--S--R--H and the gold compound of formula Au--S--R--X is such that H--S--R--H and H--S--R--X represent a compound selected from the group consisting of: N-2-(mercaptopropionyl)glycine; N-3-(mercaptopropionyl)glycine; N-2-(mercaptoacetyl)glycine; 2-mercaptoacetyl-L-histidine; 3-(mercaptopropionyl-L-histidine; L-cysteine; acetylcysteine; and glutathione.
 7. The transfer of claim 1 wherein the composition contains 8-12% by weight of the precious metal as the thiolate.
 8. The transfer of claim 1 wherein the resin of said composition comprises a member selected from the group consisting of polyvinyl pyrrolidone, polymethacrylic and polycellulose ether resin, the majority of which volatilizes in the range of 200°-550° C.
 9. The transfer of claim 1 wherein the composition contains 5-45% by weight of the resin.
 10. The transfer of claim 1 wherein the co-solvent of said composition has a boiling point between 100° and 220° C.
 11. The transfer of claim 1 wherein the co-solvent of said composition comprises water-miscible alcohol, ester or ether.
 12. The transfer of claim 11 wherein the alcohol comprises 1,2-propanediol or 1,3-propanediol.
 13. The transfer of claim 1 wherein the composition contains 5-45% by weight of the co-solvent.
 14. The transfer of claim 1 wherein the composition produces a bright film on firing.
 15. The transfer of claim 1 wherein the composition contains in addition precious metal powder or insoluble precious metal compound to produce a burnished film on firing.
 16. The transfer of claim 1 wherein the composition contains in addition an insoluble matting agent to produce a self-burnished film on firing. 